There's so many more reasons than just power to weight ratio that explain why the F40 can't keep up with modern, similar cars. Stotty mentioned one, epic turbo lag. There are many more reasons, such as aerodynamics; yeah, the F40 looks slippery, but is it? The placement of components within the car affect this, too, like radiators and intercoolers, for example, if you have a low-drag NACA duct with a big, flat radiator behind it acting like an air brake, then yeah, the air flow through the car will take a hit, and the technology behind this has changed a lot since the F40 was designed with the advent of bigger, more powerful computers and so on.
Tyres, too. I don't know a lot about tyres but it's not hard to imagine that with recent advances in tyre compounds, a 255-section tyre in 1980 would have substantially less grip than a 255-section tyre today.
Transmission technology, too; How fast can a 458 Italia shift now? is it 80 milliseconds? I was reading Evo magazine a while back and they showed the acceleration traces of a 6 speed manual compared to a 6 speed DSG gearbox, and it's stunning, DSG is more or less a smooth curve whereas the manual has big canyons between gears (as you'd expect), which is another reason why you can't compare an F40 to a modern supercar with all it's electronics.
As far as I'm concerned, the F40 is in a league of it's own. Only the Porsche 959 came close at the time, and I can't think of anything even similar to either of them from the 80's... Can anyone else?